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considerations to take into account with USB redirection, including some limitations.

One of the biggest knocks against Microsoft Hyper-V virtual desktops was the lack of USB support, but the RemoteFX add-on for Windows Server 2008 R2 Service Pack 1 (SP1) finally offers this capability. When a user plugs a USB device into a PC that's acting as a thin client, Microsoft RemoteFX makes the USB device available to the user through his virtual desktop.

The following are four factors to consider when using Microsoft RemoteFX USB redirection.

1. Hardware and software requirements

In Windows Server 2008 R2 SP1, RemoteFX installs as part of the Remote Desktop Virtualization Host role. Older versions of Windows Server do not support Microsoft RemoteFX.

These are the other requirements you need to follow for Microsoft RemoteFX USB redirection to work properly:

Virtual desktops must run Windows 7 with SP1.

Hyper-threading must be enabled in the RD Virtualization Host server's BIOS.

You must enable the Windows Aero desktop experience.

The virtual machines should contain drivers for any USB devices you may use in the remote desktop session.

The computers through which users connect to remote desktops must run version 7.1 or higher of the Remote Desktop Client software.

2. Security

Enabling USB device redirection on virtual desktops may compromise security. The lack of USB support for virtual desktops has often been regarded as a good thing, because it reduces the chance of data leaks or a user installing unauthorized software.

3. Network connectivity

Technically, Microsoft RemoteFX USB redirection should work on any TCP/IP network (assuming that the necessary virtual desktop infrastructure is in place). According to Microsoft, however, the USB redirection software is optimized for local area network (LAN) connections with latency of less than 20 milliseconds. Higher latency connections may be acceptable for USB storage devices, but that could cause problems for other types of USB devices.

4. USB redirection limitations

There are some limitations of Microsoft RemoteFX USB redirection that could cause end user confusion and an increase in support calls.

One of the first things users need to know about RemoteFX USB redirection is that the USB device must be plugged into the client computer before the user establishes a Remote Desktop Protocol session. If the user establishes a session and then plugs in a USB device, the device will not be redirected. Likewise, you can only use a redirected device in a single session.

More on Microsoft RemoteFX:

In addition, you cannot locally access USB devices once they're redirected. If a user needs local access to a USB device, they have to terminate the session to regain local device access.

Microsoft RemoteFX supports the most common USB hardware. Users should have no trouble using USB flash drives, hard drives, printers or smart-card readers. However, there are significant restrictions around some USB hardware devices, including:

Biometric devices are supported while working within a session, but they cannot be used to log in to a session.

Webcams and Voice over IP phones are supported, but they tend to be very sensitive to latency. These devices may not work correctly over slow connections.

CD and DVD drives are supported, but users can't write data to a CD or DVD.

USB displays and USB network adapters are blocked and cannot be redirected.

Some PDA and mobile devices cannot be used with Microsoft RemoteFX USB redirection.

Finally, with RemoteFX USB redirection, you must use a single redirection method for all devices. For example, CD, DVD and hard drive redirection all use a redirection method known as drive redirection. However, USB keyboards use a different method called input redirection. That means you can't simultaneously redirect a USB keyboard and a USB hard drive within a single session. Even if two or more devices do support common redirection methods, using multiple devices at the same time could result in increased latency.

Despite some limitations with Microsoft RemoteFX USB device redirection, the feature is useful for organizations that have struggled with the need for USB devices in remote desktop environments.

About the author:Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server and IIS. He has served as CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. As a freelance technical writer, he has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies.

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